Microcellular foaming of amorphous rigid polymers, polymethylmethacrylate (PMMA) and polystyrene (PS) was studied in supercritical CO2 (ScCO2) in the presence of several types of additives, such as triblock ( styrene-co-butadiene-co-methylmethactylate, SBM and methylmethacrylate-co-butylacrylate-co-methylmethacrylate, MAM) terpolymers. This work is focused in the two-step foaming process, in which the sample is previously saturated under ScCO2 being expanded in a second step out of the CO2 vessel (e.g. in a hot oil bath) where foaming is initiated by the change of temperature near or above the glass transition temperature of the glass/polymer glassy system. Samples were saturated under high pressures of CO2 (300 bar), at room temperature, for 16 h, followed by a quenching at a high depressurization rate (150 bar/min). In the last step, foaming was carried out at different temperatures (from 80 degrees C to 140 degrees C) and different foaming times (from 10 s to 120s). It was found that cellular structures were controlled selecting either the additive type or the foaming conditions. Cell sizes are ranging from 0.3 mu m to 300 mu m, and densities from 0.50 g/cm(3) to 1 g/cm(3) depending on the polymers considered.Read less <